This invention relates to a current-regulator circuit of an electromagnetic flowmeter for generating a supply current for an excitation circuit.
Such flowmeters, as is well known, serve to measure the volumetric flow rate of an electrically conductive fluid flowing through a flow-sensing tube of an electromagnetic flow sensor. By means of a magnetic system connected to an excitation circuit, a magnetic field is produced which cuts the fluid within a measured volume in sections, particularly in the region of high fluid velocity, perpendicular to the direction of fluid flow and which closes essentially outside the fluid. To guide and effectively couple the magnetic field into the measured volume, the magnetic system commonly comprises two coil cores that are positioned at a given distance from each other, particularly at diametrically-opposed positions, along a circumference of the flow-sensing tube and whose respective free end faces are disposed opposite each other, particularly mirror-symmetrically. By means of at least one excitation coil of the excitation circuit, the magnetic field is coupled into the coil cores in such a way as to cut the fluid flowing between the two end faces, at least in sections, perpendicular to the direction of flow.
Because of the motion of the free charge carriers of the fluid in the magnetic field, according to the magnetohydrodynamic principle, an electric field of flow-rate-dependent strength is produced in the measured volume which is perpendicular to the magnetic field and perpendicular to the direction of fluid flow. By means of two measuring electrodes positioned at a distance from each other in the direction of the electric field and by means of an evaluating circuit connected to them, a voltage induced in the fluid can be measured. To pick off the induced voltage, either galvanic electrodes, which are in contact with the fluid, or capacitive electrodes, which do not contact the fluid, are used.
The evaluating circuit derives from this voltage both a measurement signal representative of an instantaneous volumetric flow rate and a measurement signal representative of a volumetric flow rate integrated over time, i.e., a totalized volumetric flow rate. These measurement signals are preferably transferred in the form of digital data to higher-level, process-monitoring and/or process-controlling information systems. To that end, the flowmeter is connected to a suitable data transmission system.
During operation of the flowmeter, the excitation circuit is commonly supplied with a regulated direct current and/or a clocked direct current. To produce the magnetic field, this direct current is converted in the excitation circuit to an excitation current flowing intermittently or alternatingly through the excitation coil. Such intermittent or alternating excitation currents can be set, for example, by means of an H or T network which is connected to the excitation coil and cyclically reverses the polarity of the impressed direct current, cf., for instance, U.S. Pat. Nos. 4,410,926 or 6,031,740.
U.S. Pat. No. 4,410,926 discloses a current-regulator circuit of an electromagnetic flowmeter which serves to produce a supply current for an excitation circuit comprising at least one excitation coil which in operation is traversed by the excitation current, the current-regulator circuit comprising:
control electronics
to which an excitation current signal representative of the excitation current and a reference current signal representative of a reference value for the excitation current are applied at the input end, and
which provides at the output end a pulse-width-modulated first clock signal and a second clock signal; and
setting electronics
with a smoothing capacitor and
with a switch,
the setting electronics being supplied with an alternating voltage and being controlled by a first and a second clock signal, and
the setting electronics setting the supply current by means of the switch, and
the smoothing capacitor being temporarily traversed by the supply current.
It turned out, however, that if switching regulators for setting the direct current in conventional electromagnetic flowmeters are used in an electromagnetic flowmeter electrically powered via a two-wire current loop, their efficiency is too low. By means of such two-wire current loops, such as the standard interface RS-485 or a 4- to 20-mA current loop, for example, field devices, particularly intrinsically safe devices, are preferably coupled to the aforementioned data transmission system and/or supplied with operating power. For the data transmission systems, field bus systems, particularly serial systems, such as Profibus-PA, Foundation Field Bus, CANbus, etc., and the corresponding communications protocols may be used.
It is therefore an object of the invention to provide a current-regulator circuit which is suitable for use in an electromagnetic flowmeter and particularly in an intrisically safe and/or field-bus-enabled flowmeter.
To attain this object, the invention provides a current-regulator circuit of an electromagnetic flowmeter for generating a supply current for an excitation circuit comprising at least one excitation coil which in operation is traversed by an excitation current, the current-regulator circuit comprising:
control electronics
to the input end of which an excitation current signal representative of the excitation current and a reference current signal representative of a reference value for the excitation current are applied, and
which provides at the output end a pulse-width-modulated first clock signal; and
setting electronics with
a storage choke,
a smoothing capacitor,
a first switch, and
a second switch,
the setting electronics being powered from a two-wire interface and controlled by the first clock signal, and
the setting electronics setting a choke current flowing through the storage choke by means of the first switch and the second switch in such a way that
the smoothing capacitor is traversed by an alternating-current component of the choke current, and
the excitation circuit is traversed by a direct-current component of the choke current representing the supply current.
In a first preferred embodiment of the invention, the second switch is bypassed with a diode which is traversed by the choke current when the switches are simultaneously open.
In a second preferred embodiment of the invention, the first clock signal serves to control the first switch, and the setting electronics comprise a logic circuit which generates by means of the first clock signal a second clock signal for controlling the second switch.
One advantage of the invention consists in a high efficiency of the setting electronics, particularly during the discharge of the smoothing capacitor. Another advantage of the invention is the wide dynamic range of the setting electronics during the setting of the supply current.